Latest developments:

DENVER (CNN) -- FBI whistleblower and chemist Frederic
Whitehurst took the stand Tuesday in the Oklahoma City
bombing trial of Timothy McVeigh to bolster the defense
contention that the FBI lab that tested evidence was tainted
and that a key piece of evidence is unreliable.

Frederic Whitehurst testified that the lab where the bombing
evidence was examined was contaminated beforehand with
explosives residue.

Whitehurst said he led an examination of the lab before the
bombing and "found that there were some places that had four
or five, maybe that had some organic explosives
(contamination)."

He said contamination was found in the evidence handling and
storage areas, where explosives residue later was found on
Timothy McVeigh's clothing, knife and earplugs.

Whitehurst: Contamination possibility serious

The possibility of contamination was serious, yet the FBI to
his knowledge conducted no further studies, Whitehurst
testified as McVeigh's defense launched an assault on
evidence handling in the case.

In April 1995, Steven Burmeister, who testified about finding
explosives residue on McVeigh's belongings, kept small
containers of explosives in his lab, Whitehurst said.

He said RDX was found on Burmeister's computer keyboard in
early 1990, but said he did not know of any other problems
with Burmeister's lab.

In May 1995, Whitehurst said, he conducted contamination
tests in the receiving area of the FBI crime lab and found
traces of the explosives residue PETN. PETN was allegedly
found on McVeigh's clothing and earplugs.

The defense is suggesting the evidence may have been
contaminated with PETN in the lab itself.

Whitehurst's allegations about shoddy work in the FBI lab led
to a Justice Department report that was highly critical of
the lab.

Ryder truck panel evidence not reliable?

Whitehurst is also expected to testify that a key piece of
the government's evidence, a piece of the panel from the
Ryder truck used in the bombing on which ammonium nitrate
crystals were found, is not reliable evidence because it was
found by a civilian, not the FBI as prosecutors claim.

Earlier Tuesday, the defense called FBI lab scientist David
Williams, who was criticized in the Justice Department report
for drawing conclusions about the Oklahoma City bombing case,
to testify about the Ryder truck panel.

Williams was asked if he ever told Whitehurst that the panel
from the Ryder truck was discovered by a citizen and
therefore not of evidentiary value.

"I have no recollection of saying that to Dr. Whitehurst,"
said Williams, an explosives specialist assigned as the crime
scene manager an hour after the bombing.

Williams also testified that it was impossible to tell who
checked in some evidence early in the investigation because
no one's initials appeared to be on the evidence.

Last month, the Justice Department report found some FBI
scientists, including Williams, produced flawed work or
slanted their findings in favor of the prosecution.

The report recommended censure for Williams for allegedly
failing to make conclusions on scientific grounds, and
demotion for former chemistry-toxicology unit chief Roger
Martz, who actually handled some of the bombing evidence.

The defense has asked U.S. District Judge Richard Matsch to
admit the report criticizing the FBI lab.

Judge excludes ATF informant's testimony

Also Tuesday, Match ruled that former ATF informant Carol
Howe will not be allowed to testify for the defense.

Matsch ruled in a closed-door hearing that Howe's testimony
"lacks relevance to McVeigh's case and could confuse or
mislead the jury," according to Clark Brewster, Howe's
lawyer.

Howe's lawyer said earlier that she was scheduled to testify
Tuesday afternoon, and that her testimony would be "very
interesting." She had been expected to testify about audio
recordings that she made and handwritten notes she kept about
information she received concerning alleged bomb threats, the
lawyer said.

Howe was a paid informant for the ATF for more than two years
who reportedly said she warned federal authorities four
months before the Oklahoma City explosion that 15 U.S. cities
would be bombed.

Government sources have questioned her credibility, and Howe
is under indictment in a separate case in Tulsa, Oklahoma,
for alleged bomb threats and possession of a destructive
device.

Trial resumes with meterologist's testimony

The trial resumed after a three day holiday Tuesday with the
defense calling a professor of meteorology to the stand to
testify about weather conditions on April 17, 1995 -- the day
prosecutors allege McVeigh rented the Ryder truck used in the
bombing.

Anthony Rockwood testified that based on observation reports
from several weather stations, there were constant rain
showers in Junction City from late morning until well into
the evening during the time prosecutors contend McVeigh
walked the mile and a half from a McDonald's to Elliott's
Body Shop in Junction City.

Body Shop owner Eldon Elliott, during testimony for the
prosecution earlier in the trial, said there was only a light
mist that day, and he did not describe the person who rented
the truck as being wet.

During cross-examination by prosecutor Joseph Hartzler,
Rockwood acknowledged there could have been sporadic
interruptions in the showers and that only a half-inch of
rain fell in the area during the 12-hour period he
researched.

Defense lawyers are expected to wrap up their case this week.

McVeigh, 29, and his alleged co-conspirator Terry Nichols are
charged with murder and conspiracy in the April 19, 1995,
bombing of the Alfred P. Murrah Federal Building that killed
168 people and injured hundreds more. If convicted, the Gulf
War army veteran could receive the death penalty.